This invention relates to a current switching circuit using field effect transistors and a diode pair, which is applied to analog and digital integrated circuits for communication, computer and measuring systems.
Recently, analog integrated circuits and digital integrated circuits, which can process signals at a speed of several gigabits per second (Gbit/sec) or several giga Hertz (GHz), are required more increasingly in connection with the recent upgrading of signal processing time in communication, computer, measuring systems and so forth. As devices realizing integrated circuits operating at Gbit/sec bands, GaAs FETs (Field Effect Transistor) which replace conventional Si transistors, are very promising, and development of analog integrated circuits and digital integrated circuits using these FETs has been being carried out actively.
On the other hand, a current switching circuit consisting of a differentially connected transistor pair is one of basic circuits for realizing analog integrated circuits and digital integrated circuits capable of several gigabits per second (Gbit/sec) or several giga Hertz (GHz) signal processing. This current switching circuit is particularly promising as a circuit for realizing differential amplifiers, logic gates, and so forth. Recently it has been applied to circuits, for which ultra-high speed large current switching is required, such as laser diode driver circuits for optical communication.
However, in order to achieve the increased gain of differential amplifiers or ultra-high speed large current switching of logic gate circuits, laser driving circuit, etc., it is necessary to make the transconductance of the current switching circuit as large as possible. In order to enlarge the transconductance, it is important to reduce the amplitude of the input signal, which is necessary for perfectly switching the current, and this was a significant technical subject to be solved for realizing analog integrated circuits and digital integrated circuits operating at Gbit/sec bands.
For the differential connected transistor pair described above constituting the current switching circuit, in general, bipolar transistors or FETs are used. For example, FIG. 1 shows a circuit diagram of an FET current switching circuit, which has been heretofore widely utilized in differential amplifiers, logic gate circuits, semiconductor laser driving circuits and the like (e.g., see Technical Report of the Institute of Electronics and Communication Engineers of Japan, OQE 83-58, Aug. 25, 1983). The current switching circuit using FET has an advantage in that its production process is simple because of its lateral structure. This circuit switches the current of a constant current source 3 by means of a current switch consisting of source-coupled FET 1 and FET 2, depending on the potential difference between two complementary input signals, which are supplied through terminals 10 and 12, respectively, in order to supply the current to load circuits (resistors or FETs for differential amplifiers or logic gate circuits and lasers for semiconductor laser driving circuits) 4 and 5, and to take out the output (light output for the laser driving circuits) from terminals 21 and 22. However, FETs have a disadvantage that their mutual conductance is smaller by about one order of magnitude than that of bipolr transistors and that large input signal amplitude is necessary for switching the current because they operate based on square rule in their voltage-current characteristic.
In FIG. 1, representing the current flowing through FET 1 by i.sub.1, the current of the constant current source 3 by I.sub.0, the potential difference between the two input signals at the terminals 10 and 12 by v.sub.i, and the conductance factor of the FETs 1 and 2 by K, the switching characteristics can be given by the following equation: ##EQU1## From Eq. (1) the input signal amplitude necessary for completely switching the current is ##EQU2## which means that the larger the current to be switched, the larger is the input signal amplitude necessary for switching the current. For example, suppose that the current to be switched is 20 mA (e.g. in a laser driving circuit) and K is 20 mA/V.sup.2. Then, the input signal amplitude necessary for switching the current is 2 V.sub.p-p, which is very large.
FIG. 2 indicates a current switching circuit, wherein the pair of differentially connected transistors is constructed by bipolar transistors 101 and 102. This circuit has a large mutual conductance, because the collector current of the transistors 101, 102 varies exponentially with respect to the base-emitter voltage, and it has, therefore, a remarkable feature that an input signal amplitude less than 0.2 V is sufficient for switching the current. However, since the bipolar transistors should be constructed in a vertical NPN structure, in the case where a substrate of GaAs is used, it is very difficult to enlarge the integration scale and to realize ICs with a high production yield.